<?xml version="1.0" encoding="utf-8"?>
<grammar xmlns="http://relaxng.org/ns/structure/1.0">
  <define name="book-content">
      <attribute name="id"/>
      <attribute name="available">
        <ref name="available-content"/>
      </attribute>
      <ref name="isbn-element"/>
      <ref name="title-element"/>
      <zeroOrMore>
        <ref name="author-element"/>
      </zeroOrMore>
      <zeroOrMore>
        <ref name="character-element"/>
      </zeroOrMore>
   </define>
</grammar>
